1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a printer, and more particularly to an image forming apparatus having a unit for fixing a non-fixed image.
2. Related Background Art
Hitherto, a major portion of an image forming apparatus typified by a laser beam printer has mainly comprised a developing unit for developing a latent image formed by a laser beam, a transferring means for transferring a developed toner image onto a sheet member, and a fixing unit for fixing the transferred toner image onto the sheet member. The fixing unit for fixing the transferred toner image to a paper sheet has usually comprised a heater, a fixing roller which is rotated while being heated to a predetermined level under control of a temperature sensor, and a pressure roller which is brought into contact with the fixing roller under predetermined pressure. The pressure roller follows the rotations of the fixing roller. The unit is adapted to a method in which toner serving as a developing material is melted by heat and pressure to fix the toner image to the sheet material.
FIG. 12 illustrates a laser beam printer that is a background of the present invention.
The printer body (the body of the image forming apparatus) has an upper portion 112 that comprises: a laser scanner 102; a process unit 101 including a photosensitive drum 163, a primary charger 162 and a developing unit 164; and paper discharge rollers 103. In a lower portion 111 of the printer body there is contained a transferring roller 109, a fixing unit 104, a pair of conveyance rollers 108, a paper feeding cassette 106, and paper feeding rollers (pickup rollers) 107.
The laser beam printer has expendable units, such as process unit 101 and the fixing unit 104, and expendable rollers, such as the paper feeding roller 107 and the transferring roller 109. The expendables must be changed several times during the life of the apparatus. In particular, the fixing unit must be changed two to four times during the life of the apparatus. Therefore, the fixing unit can be changed by opening the image forming apparatus in an alligator manner as shown in FIG. 12.
FIG. 13 is a perspective view of the fixing unit shown in FIG. 12.
The fixing unit 104 shown in FIG. 13 has a bottom plate 116 that is secured to the body of the apparatus by four screws 115. The fixing unit 104 can be removed from the body by removing the four screws 115 so as to be changed.
However, an apparatus of the foregoing type requires use of a screwdriver to change the fixing unit because the bottom plate of the fixing unit is secured to the body frame by screws. Furthermore, the change cannot easily be completed because many screws are used.
Since the fixing unit is secured to the body only by the bottom metal plate, the upper portion of the fixing unit is unstable, causing the upper portion thereof to encounter resonance from vibration of the rollers when rotating. The vibrations cause the drive transmission system of the apparatus to rotate irregularly. Thus, there arises a problem, for example, in that irregular pitch halftone horizontal lines are formed on the image. In order to overcome the foregoing problem, it might be considered feasible to increase the number of fixing screws or thicken the metal frame of the fixing unit. However, any of the foregoing countermeasures raises a problem that the cost and the weight cannot be reduced.